I lean towards "no" on tipping for a six-month old. Why? A six-month old isn't going to be feeding himself; thus, there won't really be much of a mess to clean up. At six months, my daughters were mainly on breastmilk, and they only had a few spoonfuls of veggies here and there. Now if we were talking about 12 month olds, that'd be a different story. When my girls were 6 months old, they never ate IN a restaurant. They always ate JUST BEFOREHAND so that my hands would be free during the meal. Having just had a meal, they'd be very happy during dinner. I'd take along toys, etc. for them, but they never had food in a restaurant at that age.
If your habits are like mine, I'd say there's no need to tip for a six-month old. If yours are different, adjust accordingly.